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The Curious Case of the Chromospherically (and Isochronally) Candescent K Stars

Published onJun 01, 2020
The Curious Case of the Chromospherically (and Isochronally) Candescent K Stars

Astronomers are presently exploiting the potent combination of color-magnitude diagrams and Galactic kinematics constructed from Gaia Space Astronomy Mission data to attempt to obtain a complete census of young, low-mass stars within ~100 pc, a volume that excludes the nearest-known regions of active star formation. The resulting Gaia-based census of nearby, young stars will represent a fundamental resource with which to investigate the early evolution of stars and planetary systems and the dispersal of young clusters into the field. But the Gaia color-magnitude and kinematic data have already yielded some intriguing puzzles. In particular, it appears that a significant fraction of apparently isochronally young (pre-main sequence), chromospherically active (UV-luminous) K and early-M stars have anomalous (non-young-star-like) kinematics. These stars are representative of a domain of spectral classes that is central to direct-imaging exoplanet searches and habitable exoplanet studies. For most of them, however, the various commonly applied age diagnostics (e.g., isochrones, magnetic activity, Li absorption lines) appear discrepant or are otherwise inconclusive, even when potential complicating factors (e.g., binarity) are accounted for. Here, we demonstrate how the combination of (GALEX-based) UV excesses, X-ray fluxes, and newly available rotation rates from the TESS mission can potentially serve to assess the youth of these candidate pre-main sequence K and early-M field stars. This approach holds promise for purposes of distinguishing between “bona fide” young, low-mass stars and “imposters” — “overluminous”, UV-bright ZAMS or evolved main sequence stars — and could shed light on why the latter might masquerade as the former.

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